1. Field of the Invention
The present invention relates to a process for purifying phosphoric esters, and particularly to a purification method for preparing phosphoric esters which have low acid value and are excellent in physical properties such as heat resistance, storage stability and resistance to hydrolysis.
2. Description of the Prior Art
Known methods of synthesizing phosphoric esters include a method by dehydrochlorination of phosphorous oxychloride with alcohols or phenols. With such synthetic methods, however, complete esterification cannot be attained with the resulting that the synthesized phosphoric esters show some acid values resulting from the phosphoric acid or its chloride which is as the starting material.
Therefore, in general, in order to obtain the phosphoric esters having low acid values, neutralization with basic substances including wet neutralization with an alkaline metal hydroxide such as sodium hydroxide and dry neutralization with an alkaline metal compound such as calcium carbonate and magnesium hydroxide is conducted, followed by washing with water and distilling to purify the phosphoric esters.
However, where phosphoric esters having a high viscosity are purified by the wet neutralization with alkaline metal hydroxide, due to the difficulty in separation of aqueous layer and oily layer, the treating time is long and a slight amount (e.g., several to hundreds of ppm) of the alkaline metal remains in the oily layer after the separation. The alkaline metal remaining in the purification process of the phosphoric esters disadvantageously lowers heat resistance and resistance to hydrolysis of the phosphoric esters.
Various attempts have been made to decrease the remaining amount of the alkaline metal. These attempts include diluting the phosphoric esters with an organic solvent to lower the viscosity and salting-out to readily separate the aqueous layer from the oily layer. Those attempts, nevertheless, fail to prevent the slight amount of the alkaline metal from being present in the product layer. For this reason, the phosphoric esters are washed many times with water to remove the alkaline metal. The same problem applies to the dry neutralization.
Also, the wet neutralization with alkaline metal hydroxide is not applicable to some of the phosphoric esters, leading to emulsification of the whole solution of the phosphoric esters and insufficient separation of the aqueous layer from the oily layer.
The phosphoric esters can also be purified by distillation, and such a purification technique by distillation is applicable to the phosphoric esters having low molecular weight without the problem of alkaline metal remaining in the oily layer. With this technique, however, distillation apparatus are required such as fractionating apparatus having a good fractional distillation effect to remove impurities other than the alkaline metal which may reduce the physical properties (e.g., heat resistance, storage stability and resistance to hydrolysis) of the phosphoric esters, and increasing the molecular weight of the phosphoric esters makes it difficult to purify the phosphoric esters by distillation. In addition, the low yield with the purification technique leads to higher cost of the phosphoric esters to be obtained.
In addition to the remaining alkaline metal in the purification process, the impurities causing the lowering of the heat resistance and resistance to hydrolysis of the phosphoric esters include compounds not completely esterified such as compounds having the phosphoric acid or alcohols bonded to catalysts, and other impurities in a slight amount. Those impurities cannot be completely removed by the above described purification technique such as neutralization and distillation. The purification technique can produce the phosphoric esters having low acid value but not the phosphoric esters excellent in heat resistance, resistance to hydrolysis, storage stability or the like.
Japanese Patent Publication No. Hei 1-52379 describes a purification method of a dialkyl maleate, comprising the steps of neutralizing an acid component in a reaction solution containing the dialkyl maleate obtained by reacting maleic acid with an alcohol, with a basic substance such as sodium hydroxide and potassium hydroxide; heating the resultant at an elevated temperature; and reneutralizing with the basic substance an acid component generated by the heat treatment; and distilling the resultant.
According to the description of the publication, the method is characterized in that a purification technique by distillation becomes available by decomposing the impurities into the acid components using heat treatment and then by reneutralizing the resultant.
In purifying the phosphoric esters in accordance with this method, however, the basic substance remains in the ester prior to distillation because of the neutralization process included in the method, making it impossible to employ the method for the ester easily to be emulsified as described above. In addition, a problem with this method is that the impurities cannot be removed, without distillation.